Polyether siloxanes are employed in a multitude of industrial processes, for example, as defoamers in fuels, as an additive in paints and coatings, and as a constituent of cosmetic formulations. Polyether siloxanes are also suitable for use as a polyurethane foam stabilizer. A multitude of different polyether siloxanes are marketed, for example, by Evonik Industries AG under the trade name Abil®.
Of particular importance are polyether siloxanes comprising polyether radicals linked to a siloxane backbone via SiC functions. Such polyether siloxanes may be produced by hydrosilylation of polyethers comprising terminal C—C double bonds with SiH-functional siloxanes. C—C double bond-containing polyethers may be produced, for example, by alkoxylation of allyl alcohol and are marketed as allyloxypolyethylene glycols. Typical representatives of this material class are, for example, those having CAS numbers 27274-31-3, 9042-19-7 and 9041-33-2.
The production of polyether siloxanes by hydrosilylation is a known process and has been described many times in the literature, for example in U.S. Pat. No. 7,157,541 and U.S. Patent Application Publication No. 2005/0075468. The catalysts typically employed for hydrosilylation are platinum compounds. In commercial practice the use of hexachloroplatinic acid and Karstedt's catalyst and/or formulations thereof has become established for this purpose.
Hydrosilylation is accompanied by a plurality of side reactions. In the presence of OH functions, dehydrogenative coupling of the OH group-bearing component and the SiH-functional siloxane takes place and SiOC functions are formed. Additionally, in the hydrosilylation of allyl group-containing compounds, rearrangement reactions and cleavage reactions occur and propionaldehyde is formed. Propionaldehyde is one component that can result in polyether siloxanes having a strong intrinsic odor.
The aldehyde liberated can further bring about linkage of two polyether siloxane molecules by reacting with OH groups belonging to the polyether radicals and bridging the radicals via acetal bridges.
The formation of SiOC functions and acetal bridges and further crosslinking reactions are generally unwanted since they result in the build-up of highly crosslinked structures. This results in increased product viscosities which markedly hampers processing of the polyether siloxanes and may impair performance. Depending on the extent of crosslinking, gel formation may even occur.
There are a multitude of patent documents concerned with controlling these side reactions.
U.S. Pat. No. 4,847,398 (Union Carbide Corporation, 1989) describes a solvent-free process for producing polyether siloxanes in the presence of a carboxylic acid or a carboxylic acid salt. The use of such additions controls the formation of polyether siloxanes bridged via acetal groups. The examples in U.S. Pat. No. 4,847,398 describe the use of a 3.3% solution of H2PtCl6 in 1,2-dimethoxyethane and ethanol (w(Pt)=1.6%). The concentration of the assistants is in the range of from 200 to 10 000 ppm.
EP 2463291 (Shin-Etsu, 2012) describes hydrosilylation in the presence of a carboxamide, a mixture of a nitrile component and an aromatic hydroxy component or a carboxamide salt. The additions are added with the aim of improving the selectivity of the hydrosilylation. The teaching of EP2463291 warns against the use of tertiary amines because they act as a catalyst poison.
DE 102009027215 discloses hydrosilylation with Pt(0) complexes in the presence of amine-N-oxides. Pt (0) catalysts having a platinum content in the range of 0.5-5% are particularly preferred in DE 102009027215.
EP 0032377 (Union Carbide, 1992) describes hydrosilylations in the presence of sterically hindered amines and phosphines. Sterically hindered amines are considered to be amines possessing at least one alkyl radical which comprises a secondary or tertiary carbon atom bonded directly to the amine nitrogen. It is further described that the addition of such amines controls side reactions without impairing the reactivity of platinum catalysts. A series of non-inventive examples shows that various tertiary amines, such as triethylamine, markedly reduce the reactivity of the platinum catalyst.
DE 102009027215 describes hydrosilylation of olefins with SiH group-comprising compounds in the presence of Pt(0) complexes and at least one amine oxide.
The reduction in catalyst activity is an unsolved problem for those skilled in the art. Moreover, the sometimes very high viscosity of the hydrosilylation products, which is caused by the long reaction time of catalyst systems having a relatively low activity, has not been satisfactorily avoided either.